Keep in mind that this answer is coming from a contractor and some speculation from what I have heard from the other Scabs that have or are planning on leaving.
The number one reason is money. Contactors go on the road for one reason only and that is to make as much money as possible in the shortest amount of time. When a contractor shows up on a site he/she expects to work sixty plus hours. That simply isn't happening here plus NWA wants to drag forty hours out of a six day work week. That just rubs these guys the wrong way. I have tried my damnedest to adjust to this but it simply isn't working for me.
Vacation is another issue. Most of these guys can't tell you what they are doing a week from now yet NWA wants them to bid their vacations a year out. If these guys have time on the books they want to take it when they want and not be dictated by some seniority date. This was brought up about three months ago and again in our last big Pow-Wow. NWA says they are working on a solution but this brings up yet another problem. Things happen very quickly in the contract industry so contractors become very impatient when it comes to a company dragging their feet. It has been three months now and NWA is no closer to a solution than they were three months ago. Contractors look for results not petty pep talks.
Lies is also a big issue. When we hired on we were told that we would not be part of AMFA. Yet when problems arise that we want corrected they say their hands are tied because of the AMFA Contract.

That is the very last thing an NWA Contract Scab wants to hear. NWA is having a very hard time dealing with its new work force on an individual basis, it is going to cost them.
Attendance/flight benefits. It is not very smart to chew a Scab out because he was late after bouncing all over the country trying to get back for the past 24 hours because your aircraft were either full, broke or diverted. This has happened to most every Scab here at least once. It does nothing but set up discontent throughout the entire workforce that grows into something that it shouldn't. This is just something the company needs to learn to suck up.
Housing/schools is yet another issue that a lot of the guys are having problems with. The price of a decent house here in DTW is outrageous. Some have brought their wives up here to check out the local housing market. What would be available to them is deplorable at best or such a downgrade from their existing houses that the wives simply say, "Hell no. If you want to stay here fine but me and the kids are staying home." It's then just a matter of time before the contractor hits the road again so he can spend time at home.
To wrap this up it is simply a matter of NWA attempting to bend the Scabs to its will and it simply isn't working. If NWA wishes to have a non-union work force they are going to have to learn to work with non-union people. NWA is used to dealing with sheep, union sheep that is. Give the sheep a contract bargained by their union and they will perform to that contract. It does make things really simple. They have yet to comprehend that contractors are a bunch of mavericks and has yet to learn how to deal with them. Add to the fact that they are incapable of making decisions in a decent amount of time, that by the time they do come up with a decision it is much too late.